The present invention relates to gas turbine aircraft engines and, more particularly, to gas turbofan engines in which the power extraction of the compressor and fan portions can be modulated.
It is well understood that gas turbine aircraft engines are designed to operate most efficiently at one particular condition at which the individual components are optimized and matched. Any off-design operation suffers an efficiency penalty. With the trend toward multimission aircraft with a variety of operating cycles it becomes desirable both economically and from a performance standpoint to tailor the engine operating characteristics accordingly. Current gas turbine aircraft engines tend to be somewhat inflexible in their ability to match component characteristics for varying operating modes, though some notable advances have been developed toward that end. These include the incorporation of variable compressor and fan stators, variable inlet guide vanes, bleed valves, variable area turbine and exhaust nozzles and variable pitch fan blades.
In a high bypass turbofan engine having a large fan diameter, the fan moves many times the amount of air received by the core engine. It is appreciated that during flight conditions the most efficient practical propulsive system is one in which the exhaust velocity is approximately twice that of the vehicle being propelled. Thus, during low speed subsonic flight, it is desirable to move large quantities of air at relatively low velocities, and the turbofan is well adapted for this purpose. As speed increases, the optimum engine cycle approaches that of a turbojet which moves a relatively smaller amount of air, but at a much higher velocity. What is needed is a single engine which can be modulated to perform efficiently throughout these varying flight conditions by properly matching and modulating component characteristics. Modulating the rotational speed relationship between components is an effective way of accomplishing this objective.
Furthermore, the fan rotational speed is limited to a degree by the tip velocity and, since the diameter is very large, rotational speed must be very low. The core compressor, on the other hand, because of its much smaller tip diameter, can be driven at a higher rotational speed. Thus, the necessity for separate turbines for the fan and core compressor on existing gas turbofan engines. Since the turbine is most efficient at high rotational speeds, the lower speed turbine driving the fan requires additional stages to extract the work necessary to drive the fan. These additional stages result in weight penalties which are undesirable in aircraft applications. It is desirable, therefore, to provide an engine configuration in which the turbine weight is minimized.